This invention relates to vertical seismometers.
Seismometers of the kind with which the present invention is concerned essentially comprise a mass freely supported by a spring from a base which, in use, is in contact with the earth's surface. Movements of the earth's surface are measured as movements of the mass relative to the base.
Until fairly recently, seismological instruments were designed to cover one of two distinct signal bandwidths, viz short period (say 0.3-2 seconds period) and long period (say 15-40 seconds period). Short-period recording has never presented substantial difficulties. Long-period recording, however, has required seismometers having a long natural period (i.e. the resonant oscillation period of the mass and spring) of 20-30 seconds, and such long-period instruments tend to be awkwardly large.
In the last few years interest has been shown in obtaining a single instrument to cover the complete bandwidth of 0.1-100 seconds period. Moreover the introduction of electronic feedback techniques has made possible the design of small wideband seismometers. These developments were foreshadowed in a paper "Developments in Seismometry" by M. J. Usher in J. Phys E: Sci Instrum 6, pp 501-507 (1973), to which reference may also be made for a general discussion of the theory and practice of seismometry. More recently, the same author with I. W. Buckner and R. F. Burch has published a paper "A miniature wideband horizontal-component feedback seismometer" (J Phys E: Sci Instrum 10, pp 1253-1260, (December 1977)), which describes a small seismometer and associated electronic circuits which were developed in pursuit of the above objective.
Briefly, in a feedback seismometer such as that described in the second of the above papers, the movement of the mass relative to the base is detected by a transducer and a force proportional to eg the relative displacement, velocity or acceleration is fed back in such a sense as to oppose the relative movement. As in the instrument described in that paper, the signal-detecting transducer may be capacitative and the restoring force may be applied to the mass by means of an electromagnetic force transducer. Instead of requiring a natural period of, say, 20 seconds for long-period measurements as hitherto, by such means a comparable performance can be obtained using a seismometer having a natural period of only about 1 second, although the longer the better, as discussed by M. J. Usher, C. Guralp and R. F. Burch in "The design of miniature wideband seismometers" (Geophys J. R. astr Soc (1978) 55, pp 605-613).
The present invention provides a vertical seismometer suitable for use with an electronic feedback system similar to those described in the second and third of the above papers.
It comprises a novel form of spring syspension for the inertial mass which allows the following desiderata to be met:
(a) A natural period of at least 1 second in order to produce a sufficient relative displacement of mass-to-input ground acceleration ratio of the sensing mass. (This ratio is proportional to the square of the period). PA1 (b) Avoidance of excessive stress cycles in the spring to prevent degradation of its properties. PA1 (c) Mechanical compactness to allow a vertical seismometer to be lowered down a relatively narrow bore-hole, for example of 4-inch diameter. PA1 (d) Mechanical ruggedness to withstand field operation and outdoor maintenance with non-specialised tools. PA1 (e) Spurious frequencies of oscillation of the suspension above the seismic frequencies of interest to avoid unwanted coupling of vibrations. PA1 a mass-carrying horizontally-extending boom pivoted to a rigid frame so as to move about a fixed horizontal axis; PA1 a pre-stressed flat spring having one end rigidly connected to the boom adjacent said fixed axis so that movement of the boom applies a turning moment to the spring at said one end thereof; PA1 means for locating the other end of said flat spring laterally relative to its longitudinal axis; PA1 and means for applying a substantially constant force at said other end along said longitudinal axis, said longitudinal axis intersecting said fixed axis so that said force applies substantially zero turning moment to the boom.